The invention relates to a gas generator.
Gas generators which only operate with pyrotechnic solid propellant generate extremely hot gas. It is aimed that gas generators generate gases with a low temperature, in order to reduce the thermal stress of the vehicle occupant restraint system, which is arranged downstream of the gas generator. For this reason, so-called hybrid gas generators have been developed, in which in addition to solid propellant filling a combustion chamber also a separate chamber (not defining or co-defining the combustion chamber) is present which is filled with compressed gas. The hot gas and the compressed gas become mixed on activation of the gas generator, so that the hot gas is cooled. However, such hybrid gas generators require a very large structural space. These housings of the hybrid gas generators, constructed so as to be pressure-tight, are characterized by a high weight and unfavorable structural sizes owing to the high strength requirements. In addition, these pressure-tight housings require a number of gas-tight seals at component transitions, which requires relatively high manufacturing costs.
It is an object of the present invention to provide a gas generator occupying a small space, which with the same structural volume provides a higher output than a corresponding purely pyrotechnic gas generator. In addition to the reduced structural space, the gas generator provided according to the invention is reduced in its overall weight, i.e. is lighter and provides the possibility of manufacturing various gas generators within one production line and according to a modular concept.
This problem is solved in a gas generator which comprises a housing which has an interior space and at least one combustion chamber filled with a predetermined quantity of solid propellant. The propellant consists of a plurality of solid propellant parts, e.g. bodies in tablet form or extruded shaped bodies, which are arranged in the combustion chamber in an irregular, i.e. chaotic manner as the propellant parts are thrown or poured into the combustion chamber. The compressed gas is stored in a predetermined quantity in the combustion chamber and the combustion chamber is constructed as a pressure chamber sealed hermetically towards outside in a non-activated state of the gas generator.
With the invention, therefore, the combustion chamber of a gas generator actually only operating with solid propellant is used for accommodating additional compressed gas. Between the propellant units or parts which are usually present in the form of tablets or other pressed forms (e.g. extruded shaped bodies) in fact sufficient empty space is still present in order to be able to fill this with compressed gas. These empty spaces add up as a whole to such a high unused volume that additionally introduced compressed gas can actually lead to an increase in output by more than 10% with the same structural space and the same weight. In addition, the compressed gas can reduce the temperature of the outflowing gas mixture and possibly even lead to an afterburning. Through a suitable coordination of solid propellant and compressed gas, very low emissions of harmful substances can be achieved, which are much lower than those of the purely pyrotechnic systems.
In fact, hybrid gas generators already exist, in which the combustion chamber is arranged in a pressure chamber surrounding the combustion chamber and the combustion chamber is open to the pressure chamber and also compressed gas is present in the combustion chamber. In these systems, tie combustion chamber is only therefore open towards the pressure chamber, however, so that no resistance is offered to the pressure wave and the compressed gas which are generated on igniting of the solid propellant, on overflowing into the pressure chamber. This resistance would in fact be present if the combustion chamber were closed by a bursting membrane with respect to the pressure chamber, as has also already been proposed elsewhere in the prior art.
In the gas generator according to the invention, which is constructed in one or more stages, however, no pressure chamber containing gas is present around the combustion chamber. The solid propellant in the gas generator according to the invention generates more moles gas than moles compressed gas are stored in it.
According to an embodiment, the combustion chamber is constructed as a separate, gas-tight can which is able to be acted upon by pressure, which can is integrated in the interior space of the gas generator housing. This gas-tight can is provided as a separate component and can be integrated according to requirements as a combustion chamber into the respective gas generator series.
The pressure-tight gas can is constructed for example from a gas-tight foil or from a thin sheet metal material. The gas-tight can follows in its outer shape the inner shape of the gas generator housing.
With the use of combustion chambers in the form of gas-tight cans, a bursting means which is arranged in the overflow zone of the gases in the gas generator housing, is dispensed with, because the gas-tight can has a sufficiently thin wall and breaks open in the region of the overflow openings.
The gas-tight can stores compressed gas with a pressure in the range of 10 to 20 bar.
The construction of the combustion chamber in the form of a gas-tight can offers the advantage that in a relatively simple manner a solid propellant can be surrounded by a fixedly defined gas mixture.
The optional equipping of gas generators with separate combustion chambers in can form with or without additional gas within a production line offers the advantage of high flexibility with manufacture of various gas generators using as many identical parts as possible.
Preferably, the compressed gas is a gas mixture containing oxygen. The oxygen opens up the possibility of a subsequent combustion.
If, in addition, the solid propellant in the gas generator is underbalanced with respect to oxygen, then the oxygen can make possible an oxidation of the resulting CO and H2 to CO2 or water with, at the same time, very low NOx values.
The compressed gas is stored with a pressure of more than 10 bar, even of approximately 200 bar in the combustion chamber. With the use of combustion chambers in the form of gas-tight cans, the compressed gas is stored at a pressure of 10 to 20 bar, i.e. a pressure which lies below the usual pressure of 240 bar for hybrid gas generators. Thereby, the strength requirements for the combustion chamber wall, constructed as pressure chamber wall, are substantially lower than in hybrid gas generators.